This invention relates to a color image forming apparatus in which a toner image is formed on a belt-like photoreceptor by the method of electrophotography and the image is transferred onto a transfer paper in order to obtain a color image.
Many electrophotographic methods and apparatuses to obtain a color image have been proposed. For example, the following method to obtain a color copy is disclosed in Japanese Patent Publication Open to Public Inspection No. 100770/1986: a latent image is formed and developed on a photoreceptor drum, wherein the number of the latent image forming operations corresponds to the number of separate colors; each time the latent image is developed it is transferred onto a transfer drum to form a multicolor image; and the multicolor image is transferred from the transfer drum onto a recording paper. In this method, it is necessary to provide a transfer drum the circumferential surface of which is wide enough for an image on the photoreceptor to be transferred. Accordingly, this apparatus is large and complicated.
The following method is disclosed in Japanese Patent Publication Open to Public Inspection No. 149972/1986: a latent image is formed and developed on a photoreceptor drum, wherein the number of image forming operations corresponds to the number of separate colors of a document image; and each time the latent image is developed the image is transferred onto a transfer paper to obtain a multicolor image. In this method, it is difficult for multicolor images to be precisely superimposed. Consequently, a color copy of high quality can not be obtained.
Another prior art method is characterized in that: latent image forming and latent image developing by color toner are repeatedly conducted on a photoreceptor so that the color toner images can be superimposed; and the the color toner image is transferred onto a transfer paper in order to obtain a color image. The basic process of the above-described multicolor image forming method is disclosed in Japanese Patent Publication Open to Public Inspection No. 75850/1985, No. 76766/1985, No. 95456/1985, No. 95458/1985, and No. 158475/1985, which were applications by the present inventors.
In this kind of multicolor image forming apparatus in which a color image can be obtained by superimposition, a plurality of developing units containing different color toners are provided around the circumferential surface of a photoreceptor drum and the photoreceptor drum is rotated a plurality of times so that the latent image on the photoreceptor can be developed.
It has been proposed that the image forming body include: a flexible belt image forming body on which the photoconductive material is coated or provided as well as a photoreceptor drum on which the photoconductive material is coated or vapor-deposited. The flexible belt image forming body, which will be called the photoreceptor belt hereafter, is stretched between a drive roller and an idle roller. Accordingly, when the photoreceptor belt is used in a color image forming apparatus, the space is effectively utilized. As a result, the apparatus can be made compact. Even when the drive roller or the idle roller has a small diameter, the photoreceptor belt can run smoothly. Consequently, when a small diameter roller is used in the belt portion of the apparatus, transfer papers can be properly separated from the belt at the small diameter roller portion so that defective paper separation can be prevented.
In a color image forming apparatus in which the photoreceptor belt is used, the image forming means such as the charging means, the image exposing means, and the developing means comprising a plurality of developing units, are provided around the photoreceptor belt. The above-described image forming means face the photoreceptor belt, wherein there is a constant gap between them.
In order to keep a constant gap between the photoreceptor belt and the image forming means, a back-up roller can be used as a support so that the photoreceptor belt can be supported when it faces the image forming means. However, when the back-up roller is used in the apparatus, it is necessary to provide many back-up rollers corresponding to the number of image forming means. When many back-up rollers are provided in the apparatus, it is difficult to keep them in parallel with the drive roller or the idle roller between which the photoreceptor belt is stretched. As disclosed in Japanese Patent Publication Open to Public Inspection No. 34576/1982, an apparatus is proposed in which the photoreceptor belt is supported by a guide member so that the belt can face the image forming means. However, it is difficult to maintain the gap between the photoreceptor belt and the image forming means. In the apparatus which is disclosed in Japanese Patent Publication Open to Public Inspection No. 102677/1985, the guide member comes into contact with the photoreceptor belt in a large area. However, when the contact area between the belt and the guide is large, the load is increased and slippage occurs between the drive roller and the photoreceptor belt.